The present invention relates generally to tubular structures such as water-jet nozzles for working articles with water jets propelled at high velocity, and to a process for the production thereof. More particularly, the invention relates to a silicon carbide tube to be used particularly for a nozzle to eject a water jet of the abrasive type containing particles of abrasives such as garnet and alumina admixed therein, and to a process For producing the tube.
As is known, a water-jet working or machining apparatus operates to eject water under high pressure through a nozzle of small throat diameter thereby to render the water into a jet of supersonic velocity and direct this jet against a workpiece material to work same. By this technique, almost any kind of material such as plastics, papers, and metal alloys can be worked. During this working or machining, there is almost no scattering of dust or generation of heat, and three-dimensional machining is also possible. In order to further increase the precision and speed of the working process, abrasive type water jets containing particles of abrasives such as alumina and garnet are also being developed.
The inner surface of the nozzle constituting the low path and orifice for discharging the water jet is subject to severe abrasion and wear. Accordingly, materials such as cemented carbide alloys, refractory hard metals, and alumina ceramics are ordinarily being used for these nozzles. There are also nozzles made of sintered skeletons of cubic boron nitride as disclosed, for example, in Japanese Utility Model Appln. Laid-Open No. 63-50700 published Apr. 6, 1988. These materials, however, are of high price. Moreover, in the present state of the art, these materials do not exhibit durability to a degree commensurately expectable from the intrinsic hardnesses of these materials.
Silicon carbide is a material next in hardness to diamond and cubic boron nitride. Moreover its precipitation by the vapor-phase synthesis method is relatively easy. Therefore, silicon carbide is a promising material for nozzles of the instant character.
Formed articles of silicon carbide of the prior art have been produced by mixing various sintering aids such as carbon, boron, and aluminum into a fine powder of silicon carbide synthesized by the Acheson process, for example, forming the mixture into the desired shape, and then sintering the same. It has been found that, when this material is used for a water-jet nozzle, it does not exhibit a durability expectable from the intrinsic hardness of silicon carbide. The cause of this disappointing result is considered to be that, because the sintering aids wear away priorly, or the bonding with the sintering aids is insufficient, the particles of silicon carbide become free and drop off, whereby the intrinsic hardness of silicon carbide cannot be amply utilized.
Another problem encountered hitherto has been the severe requirement for precision of shape because of the minute inner diameter of a water-jet nozzle of the order of 1 mm. For this reason, the forming of a water-jet nozzle from sintered silicon carbide has been thought to be difficult.
We have made a study of chemical vapor-phase synthesis processes with the view of providing a process for obtaining immediately in a desired shape a silicon carbide of high purity containing no sintering aids. As a result, we have found that a nozzle produced by such a process has a performance superior to that of a conventional sintered silicon carbide product. We have thus arrived at the present invention.